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California Association for

Medical Laboratory Technology

Distance Learning Program

An Overview of the Immune System, Part One: The Cells and Cell Surface Molecules

(Revision: April 2016)

Course # DL-978

by

Elizabeth Crabb Breen, M.T. (A.S.C.P.), Ph.D. Associate Professor

Norman Cousins Center for Psychoneuroimmunology David Geffen School of Medicine

University of California, Los Angeles

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Level of Difficulty: Basic

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CAMLT Distance Learning Course DL-978 © CAMLT 2016 Page 1

An Overview of the Immune System, Part One: The Cells and Cell Surface Molecules Course Number: DL-978

3.0 CE

Level of Difficulty: Basic

Elizabeth Crabb Breen, MT(ASCP), PhD

David Geffen School of Medicine

University of California, Los Angeles

ABSTRACT:

Immunology has become an integral part of clinical medicine and therefore, an

increasingly important aspect of clinical laboratory work. This is the first of two courses

designed to refresh and update the clinical laboratory scientist’s basic understanding of this

rapidly changing field. This course provides a description of the innate (non-specific) immune

system, and a review of the white blood cells that contribute to both innate and adaptive

(antigen-specific) immune responses. This includes a discussion of critical cell-surface

molecules, and the markers used to identify different cell types.

OBJECTIVES:

Upon completion of this course, the reader will be able to:

List physical and chemical barriers of the innate immune system

Identify the white blood cells that are not antigen-specific

Describe immunologic functions of the three antigen-specific lymphocyte subsets

List the differences in antigen presentation to CD4+ and CD8+ T cells

Identify cells of the immune system utilizing CD nomenclature

Preface/Author’s Note

As an undergraduate student in Clinical Science at San Francisco State University, I had

the privilege of being introduced to the field of immunology by Dr. Janis Kuby. It was her skill

and enthusiasm, as both a professor and an immunologist, that led to my pursuit of a Ph.D. in

immunology many years later. When I began teaching an undergraduate immunology course, I

was delighted to discover that she had written an immunology textbook which reflected both her

command and her love of the subject. Sadly, Dr. Kuby lost a long-standing battle with cancer

within weeks of completing the third edition of her text in 1997. It is a testament to the quality of

the original textbook that it has been continued in Janis Kuby’s name by teams of authors, with

the release of its seventh edition in 2013. It is with pride that I highly recommend Kuby

Immunology and use it as one of my primary references. Janis Kuby will never be forgotten by

anyone who has ever had the benefit of her knowledge and love of immunology, either face-to-

face or through her writing. With deep gratitude, I dedicate these immunology study courses to

Dr. Kuby’s memory.


LIST OF FREQUENTLY-USED ABBREVIATIONS WBCs white blood cells PRRs pattern recognition receptors

PAMPs pathogen-associated molecular patterns TLRs Toll-like receptors

DNA deoxyribonucleic acid RNA ribonucleic acid

CpG Cytosine-phosphate-Guanine (nucleotide sequence)

DAMPs damage-associated molecular patterns APC antigen-presenting cell

MHC major histocompatibility complex DC dendritic cell

NK natural killer cell µL microliter

TCR T cell receptor CTL cytotoxic T lymphocyte

TH T helper cell TREG regulatory T cell

CD cluster of differentiation

INTRODUCTION:

The immune system is not a single discrete organ or collection of tissues located in one or

a few anatomic sites. In fact, it could be considered two collaborative systems: the innate

immune system that reacts in a relatively non-specific manner, and the adaptive (or acquired)

immune system, capable of incredibly specific recognition and response (1,2). These immune

systems are composed of a variety of tissues and cell types, both fixed and mobile throughout the

body, that work together, first to try to prevent the entry of pathogens and/or foreign material

(known as antigens) into the body. Failing that, the immune systems spring into action to

recognize and respond to the presence of antigens in order to eliminate or neutralize them,

especially those associated with microbial pathogens.

I. The Innate Immune System Innate immunity is an ancient evolutionary feature, with some form found in all

multicellular plants and animals (1,2). The innate immune system consists, to a great extent, of

components that are pre-existing and ready to act prior to exposure to antigens. It is designed to

prevent the entry of and/or rapidly eliminate antigens such as pathogens, toxins, or other foreign

materials. Using a combination of soluble antimicrobial molecules, cells, and membrane-bound

receptors on the surface of cells, the innate immune system can initiate an instantaneous attack

on antigens. The innate system is capable of distinguishing between self and foreign molecules

by utilizing cellular and molecular components that recognize classes of molecules unique to

microbes. Unlike the more sophisticated (and complicated) adaptive immune system, the innate

system is not able to distinguish the more subtle differences among different foreign antigens or

to remember a previous exposure to the same antigen(s). It acts in the first few hours and days of

an exposure, before the activation of the adaptive immune system. In general, the innate immune

system can deal with most antigen exposures without ever calling upon or activating the antigen-

specific adaptive immune system. Innate immunity is considered, therefore, to be the first line of

defense. If an antigen exposure is sufficiently large, and/or involves a more virulent pathogen,

the innate immune system then serves as a trigger and an amplifier for the adaptive immune

system.

The first defenses of innate immunity are a collection of physical and chemical barriers

(1,2). Physical barriers include the intact skin and mucous production plus ciliary action on

mucous membranes throughout the body. Chemical barriers range from the acidity of the

gastrointestinal and genitourinary tracts to a host of molecules secreted by cells and tissues with


specialized activity against microbial and/or other antigens. These include a) enzymes in saliva

and tears such as lysozyme; b) an antibacterial protein produced by the skin called psoriasin, that

prevents bacterial colonization (especially by E. coli); and c) naturally-occurring anti-viral

proteins known as interferons. One of the best-known chemical barriers is the complement

cascade (the bane of many a clinical laboratory student), which acts in a sequential fashion to

convert inactive circulating proteins to active molecules. These molecules have the ability to

damage the cellular membranes of certain types of pathogens and/or facilitate pathogen/antigen

clearance by inducing inflammation and enhancing phagocytosis (as described below).

If an antigen is able to breach the physical and/or chemical barriers of the innate immune

system, it encounters the next line of defense, namely, the white blood cells (WBCs) within the

blood and tissues of the body. Some kinds of WBCs contribute to innate immunity by

recognizing patterns of molecules that are found on frequently-encountered pathogens, but are

typically never expressed on the cells and tissues of humans (or other multicellular organisms)

(1,2). The WBCs utilize sensors or receptors on their cell surfaces, known as pattern recognition

receptors (PRRs), that recognize particular types of repeating structures on microbial species.

These structures are usually necessary for survival for microbes, and therefore, tend to remain

constant across species and across time. Examples of these microbial structures, which are called

pathogen-associated molecular patterns (PAMPs), include components of bacterial or fungal cell

walls, and molecules present on or inside parasites and viruses. The first (and best understood)

group of innate system PRRs is the Toll-like receptors (TLRs), that were discovered and

characterized as part of the innate immune system in humans in the late 1990s. There are

currently ten known human TLRs, which can be categorized into two distinct groups based on

the location of each receptor on a WBC and the corresponding location of the microbial

structure(s) recognized. One group of TLRs is found on the outer cellular membrane of WBCs,

and recognizes a broad variety of molecules on the outer surfaces of pathogens, e.g., flagellin in

bacterial flagella, lipopolysaccharide (LPS) in gram-negative bacterial cell walls, and zymosan in

the cell walls of fungi. A second group of TLRs is located inside WBCs, within intracellular

compartments known as endosomes. These TLRs, which interact with molecules inside the cell,

are capable of recognizing characteristics of nucleic acids (deoxyribonucleic acid [DNA] and

ribonucleic acid [RNA]) of pathogens, which serve as PAMPs. One of the microbial molecular

structures recognized inside the cell is a specific form of a particular nucleotide sequence

(Cytosine-phosphate-Guanine, or CpG) within DNA (the genetic material of all cells). While

both human and microbial DNA contain CpG sequences, in human DNA all CpG sequences are

chemically modified by the addition of a methyl group (known as CpG methylation). Bacterial

DNA, however, is not methylated at CpG sequences. Therefore, when bacteria are phagocytosed

or are otherwise present within a cell, and are digested to reveal their DNA, one of the

intracellular human TLRs recognizes the unmethylated bacterial CpG sequences as a PAMP.

Other PAMPs recognized within a cell are forms of RNA that do not occur normally within a

human cell, but can be detected as single-stranded RNA (e.g., common cold viruses, Hepatitis C

virus) or double-stranded RNA (e.g., rotaviruses that cause gastroenteritis) associated with viral

infections. The engagement of TLRs on or in WBCs by the appropriate microbial molecules of

potential pathogens makes WBCs more efficient in processing, killing, and clearing pathogens.

The binding of TLRs to microbial structures also contributes to the development of

inflammation, and stimulates the production of signaling molecules known as cytokines that

affect the behavior of WBCs involved in both innate and adaptive immune responses.


II. The WBCs of the immune system WBCs are an essential part of both innate and adaptive immune responses. In order to

understand the workings of the immune system as a whole, it is necessary to be familiar with the

different types of WBCs and the role(s) each plays in one or both of these immune responses.

WBCs circulate not only in the blood, but also through the lymph nodes via the lymphatic

system. In addition, they are widely distributed throughout the tissues of the body, and in many

cases, move freely between blood, lymph, and tissue compartments. Because of the ease of

collection of peripheral blood compared to lymph fluid and/or tissue, WBCs were first observed

and characterized in the blood, and peripheral blood remains the routine sample of choice for

evaluation of WBCs. In such a peripheral blood sample from a normal individual, a typical WBC

count would be 7.3 x 103 cells per microliter (µL) of blood (Table 1).

If blood is collected in an untreated glass tube (red top blood collection tube), it will clot,

trapping and/or lysing the WBCs in the clotting process. Therefore, if a blood sample is being

collected for examination of WBCs, it must be treated with anticoagulants to prevent clotting and

to enable the blood to remain liquid. The anticoagulant often used in blood collection for WBC

analyses is EDTA (lavender top tube); other anticoagulants commonly used for blood collection

include heparin (dark green top tube) and sodium citrate (blue top tube). When a tube of

anticoagulated blood is allowed to separate, either by low speed centrifugation or by settling out

due to gravity, it will segregate into the three basic components of blood: plasma, WBCs, and red

blood cells (Figure 1).

Plasma, the liquid portion of the blood, will be at the top of the tube. Plasma contains all

of the nutrients, vitamins, minerals, etc., as well as the wastes that circulate in the blood.

Platelets, the very small cellular fragments that are essential to the clotting process, usually

remain suspended in the plasma as well. The red blood cells will be at the bottom of the tube.

Their function is to carry oxygen to the tissues and carbon dioxide back to the lungs. In between

the plasma and the red blood cells, in a very thin layer often referred to as the “buffy coat”, will

be the WBCs. (The thickness of this layer is exaggerated in Figure 1 simply to make it visible in

the drawing.)

Classically, the WBCs have been categorized according to morphologic differences

visible in stained cells under a light microscope. The initial subdivision of WBCs in Figure 1

reflects this by separating cells into lymphocytes and monocytes (mononuclear cells) and

granulocytes (multinuclear or polymorphonuclear cells). However, for the purposes of examining

functions of the different WBCs in the immune system, it is useful to discuss them in terms of

their ability to participate in innate and/or adaptive (antigen-specific) immune responses.

Microbial pathogens and other foreign materials are typically relatively large and

complex structures, composed of many different smaller antigens. Individual antigens have

distinctive, three-dimensional shapes that can be physically distinguished from one another by

the immune system. However, not all WBCs are equipped to make such an antigen-specific

distinction. As described above, the WBCs involved in innate responses can detect and respond

to generic molecules associated with microorganisms, but cannot recognize differences between

individual antigens on, in, or produced by those organisms. Other types of WBCs are remarkably

specific, capable of recognizing and responding to individual antigens, which gives rise to

adaptive or antigen-specific immune responses. The following review of the WBCs will consider

the different cell types starting with the less-specific cells (at the bottom of Figure 1) and

progressing upwards to the antigen-specific cells (at the top of Figure 1).


Granulocytes

Granulocytes are the only WBCs with nuclei that have multiple lobes, and so they are

described as multinuclear or polymorphonuclear cells. As their name implies, they contain

granules in their cytoplasm. The three different types of granulocytes are typically distinguished

by the staining properties of these granules—neutral, acidic or eosinic, or basic. From an

immunologic perspective, granulocytes differ not only in appearance, but also in function.

Neutrophils (also known as polymorphonuclear cells, PMNs, or polys) are the most

numerous type of WBC, making up 50-70% of the circulating WBC in a normal individual

(Table I). They have distinctly multilobed or segmented nuclei (under normal circumstances) and

the granules they contain are neutral-staining. Neutrophils are a major part of the innate immune

response, utilizing TLRs on their surface to broadly recognize pathogens and other foreign

materials that need to be cleared from the body. They also utilize TLRs and PRRs to recognize

damage-associated molecular patterns (DAMPs) on dead/dying self (human) cells and damaged

tissues (1). They will attempt to engulf and take up or phagocytose any and all types of foreign

material and organisms with which they come in contact, as well as debris (such as dead and

dying cells). One could think of neutrophils as the vacuum cleaners of the body—they are not

capable of discriminating between individual antigens, and therefore, they will always respond

by trying to pick up and dispose of any offending material that they encounter, and dispose of it.

The phagocytic activity of neutrophils is greatly enhanced by the binding of active complement

components and/or antigen-specific antibodies to pathogens/antigens, a process known as

opsonization. Opsonization by molecules produced during both innate (complement) and

antigen-specific (antibody) immune responses is just one example of how the two types of

responses can affect one another.

Once a neutrophil has successfully phagocytosed an antigen, it will kill and/or digest that

pathogen/antigen inside the cell. This is accomplished by using a wide variety of enzymes such

as peroxidase and lysozyme that are stored in its granules, and highly active molecules such as

reactive oxygen and nitrogen intermediates that are produced following the process of

phagocytosis. In addition, neutrophils are rich in naturally-occurring antimicrobial peptides

known as defensins, which rapidly kill a wide variety of microbes. Once the antigens have been

degraded within the neutrophil, the resulting bits and pieces are released back into the

bloodstream (or lymph fluid), where they are eventually filtered out by the kidneys.

After being produced in the bone marrow, neutrophils circulate in the blood only briefly

(7-10 hours), and then migrate into the tissues where they remain viable for 1-3 days. They are

attracted to sites of injury and/or inflammation by chemotactic factors, which are initially

released as a result of tissue damage and clotting. Chemotatic factors are chemical signals that

attract cells to physically move (undergo chemotaxis) toward the source of the signal. The

neutrophils will move through the blood and then move out of blood vessels, to move through

tissue toward the source of the chemotactic factors; they are usually the first cells to arrive at an

inflammatory site. In fact, the white pus that develops at a wound site is composed primarily of

neutrophils that have migrated to the site. By virtue of their sheer numbers, the neutrophils will

almost always be the first to encounter invaders. They are, therefore, a critical first line of

defense against invading pathogens and antigens. If the dose of antigen is low, the neutrophils

alone may be sufficient to clear the antigen from the body, without any further need for a

response by antigen-specific WBCs.

Eosinophils are a much rarer cell type, typically making up 1-3% of circulating WBC

(Table I). They derive their name from the acidic or eosinic staining of their granules.


Eosinophils are non-specific phagocytic cells like the neutrophils, but less is known about their

function. Eosinophilia, or increased numbers of eosinophils, is characteristic of parasitic

infections, and there is evidence that this cell type is involved in immune responses to parasites,

especially helminthic worms and other intestinal parasites. Eosinophilia may also be observed in

highly allergic or atopic individuals.

Basophils are the rarest of the granulocytes, making up less than 1% of the circulating

WBC. They are non-phagocytic cells, and are easily recognized by the heavy purple or blue

basic staining of their cytoplasmic granules, that frequently obscure the nucleus. These granules

are the key to the function of basophils, as they contain histamine and other highly biologically

active molecules that are released during allergic responses. Like neutrophils and eosinophils, the

basophils themselves are not antigen-specific. Their ultimate function, however, is dependent on

interactions with antibodies of the IgE class which are antigen-specific. When coated with IgE

antibodies specific for antigens that induce allergic responses (known as allergens), basophils

will release the potent contents of their granules when IgE binds to the relevant allergen. It is

these cells, therefore, that are responsible for the all-too-familiar allergy symptoms of hay fever,

hives, and asthma, as well as the severe, life-threatening anaphylactic type of allergic responses.

A similar cell, found in the tissues rather than circulating in the blood, is known as a mast cell.

Monocytes/macrophages

Mononuclear cells are easy to distinguish from granulocytes on the basis of morphology,

and can be subdivided into monocytes and lymphocytes. These two types of mononuclear cells

have subtle differences in their morphology, but have enormous differences in their immunologic

function. These differences are reflected in the placement of monocytes in Figure 1. Monocytes

are phagocytic cells that, like neutrophils, are very important in innate responses, but do not have

the ability to discriminate among antigens, as do some of the lymphocytes. Monocytes are placed

below the lymphocytes because they lack antigen-specific recognition, but are placed above the

granulocytes due to additional functions that help them serve as a bridge between innate and

adaptive immune responses.

Monocytes typically represent 1-6% of the WBCs in the blood (Table I), where they

circulate after being produce in the bone marrow. Many monocytes are considered

“inflammatory monocytes”, which migrate into the tissues quickly in response to infection or

other distress. Other monocytes are identified as “patrolling monocytes”, which remain in the

blood and serve as a reservoir, if needed, to move into the tissues (1). The monocytes that

migrate from the blood into the tissues are then called macrophages. Macrophages may remain

mobile within the tissue, or they may become fixed, where they take on a particular function

within a given tissue. Many types of fixed macrophages have been recognized and given special

names, e.g., Kupffer cells of the liver, alveolar macrophages in the lung, and osteoclasts in the

bone. Like neutrophils, macrophages are phagocytic cells that utilize TLRs on their cell surface

to recognize antigens and/or pathogens. TLRs can also be present within intracellular

compartments formed after phagocytosis, known as endosomes. Following stimulation via the

TLRs, these cells become activated, resulting in increased phagocytosis and a greater ability to

kill pathogens and eliminate antigens, utilizing many of the same mechanisms as neutrophils.

However, unlike neutrophils, phagocytosis by macrophages also triggers a cascade of protein

production that serves additional functions. These include enzymes, which aid in enhanced

killing and/or digestion, and complement components that participate in inflammatory reactions.


In addition, activated macrophages produce and secrete a class of proteins known as cytokines,

which play important roles in both innate and adaptive immune responses.

Cytokines are protein messenger molecules that send signals or messages between cells

(1, 2). The term “cytokine” is derived from “cyto”, meaning “cell”, and “-kine”, which indicates

movement (as in the word “kinetic”). Therefore, “cytokine” is a broad term that refers to all

proteins that send messages from one cell to another, regardless of cell type or the action of the

cytokine. There are other names that have been given to subgroups of cytokines based on cell

types and/or function, such as interleukins (“between leukocytes”) and interferons (which

interfere with virus replication).

Activated macrophages secrete a very characteristic pattern of cytokines, sometimes

referred to as monokines. These secreted proteins diffuse into the blood or lymph fluid, and send

messages to nearby cells. Some of the cytokines are chemotactic factors (also known as

chemokines) that attract neutrophils to a site of inflammation. Certain cytokines secreted by

macrophages exert their effects at great distances, similar to hormones, inducing fever by acting

on the hypothalamus. Cytokines secreted by activated macrophages also help to recruit, stimulate

and/or activate antigen-specific lymphocytes, aiding in the initiation of adaptive immune

responses. Through its secretion of proteins, especially cytokines, the macrophage serves as a

bridge between innate and adaptive responses.

There is yet another function of activated macrophages, which positions these cells as

critical links between innate and adaptive immunity. When a macrophage digests antigen that has

been brought into the cell non-specifically by phagocytosis, it does not completely eliminate the

digested antigen. Rather, it retains small peptide fragments of antigen that are then transported to

and displayed on the surface of the cell. The digested antigen is presented on the macrophage

cell surface to other WBCs that are capable of recognizing and responding to it. This antigen

presentation is absolutely required to initiate an antigen-specific immune response. So, although

the macrophage itself is not antigen-specific, it can play an essential role in the initiation of

antigen-specific responses by serving as an antigen-presenting cell (APC).

When fragments of digested antigens are presented on the surface of a macrophage, they

are physically-associated with major histocompatibility complex (MHC) molecules. These

molecules, which in humans are referred to as human leukocyte antigens (HLA) or

transplantation antigens, are the molecules that the immune system uses to distinguish self cells

and/or tissues from non-self or foreign cells. Every individual has a collection (or complex) of

MHC molecules that are identical throughout his or her body; however, one person’s collection

of MHC molecules is going to be distinctively different from those on most other individuals’

cells and tissues. Therefore, it is the collection of MHC molecules found within each individual

that defines what is self or non-self. During embryonic development, the immune system learns

to recognize a self cell by interacting with the MHC molecules expressed on cells throughout the

body. At the same time, exposure to self antigens trains the developing immune system to ignore

self components and mount immune responses only against foreign antigens. In a normal

individual, these processes result in antigen-specific WBCs that will only respond to an antigen

that is both foreign and properly presented in association with self-MHC molecules. If the

immune system fails to discriminate between self antigens and foreign antigens, then an

autoimmune condition can develop. This is one of many systems of checks and balance within

the immune system, to ensure that only appropriate responses are mounted. Therefore, the ability

of macrophages to present antigen in association with MHC molecules is absolutely critical to

their ability to act as APC.


Macrophages are not the only innate immune cells that can present antigen (1,2). There is

another population of WBCs, called dendritic cells (DC), which are non-specific, yet highly-

efficient APCs. Their name is derived from long extensions that extend outward from the body

of the cell that resemble the dendrites of nerve cells, but in these APCs, serve to increase the

surface area available on which antigens can be presented. Immature DCs are activated via TLRs

and other cell surface receptors, resulting in high levels of MHC molecules on their cell surfaces.

As with other innate cells in the peripheral blood, antigens are phagocytosed and degraded in the

circulation, but DCs then migrate to secondary lymphoid tissues (such as lymph nodes), where

they are very potent APCs for most kinds of antigen-specific T lymphocytes. DCs appear to be a

diverse population that may be produced in the bone marrow by both myeloid precursors (which

give rise to monocytes and macrophages) and the precursors of lymphocytic cells (1). DCs are

not shown in Figure 1, as they are extremely rare in the peripheral blood (0.1% of circulating

WBCs). DCs are more commonly found in the thymus, lymph nodes, and other immune system

tissues, in most organs, and in the skin, where they are called Langerhans cells.

Lymphocytes

The remaining WBCs in Figure 1 to be discussed are the other type of mononuclear cells,

the lymphocytes. Lymphocytes account for 20-40% of the total WBCs circulating in the blood

(Table I). However, the lymphocytes in the peripheral blood represent only 1% of all

lymphocytes. The remaining 99% are found in the lymph system, circulating in the lymph fluid

and residing in regional lymph nodes and other lymphatic tissue such as Peyer’s patches in the

intestine and tonsils in the throat. Because such a large percentage of these WBCs is spread

throughout the body, the immune system is often thought of as a relatively small organ system. It

is estimated, however, that in an average human being, there are 1010

- 1012

lymphocytes

distributed throughout the body, which collectively have a cellular mass equivalent to the liver or

brain.

There are three different types of lymphocytes, all of which are non-phagocytic,

mononuclear WBCs. They are indistinguishable by morphology alone, and so have historically

been identified by differences in cellular function. One of the three, the natural killer (NK) cell,

although it is a lymphocyte by hematologic lineage, is an innate non-specific cell. Hence, it is

shown in Figure 1 below the other two types of lymphocytes, and will be discussed first.

The proportion of lymphocytes that are NK cells can vary widely from person to person,

to make up anywhere from 4% to 19% of the circulating lymphocyte population (Table II). They

are called “natural killers” because of their innate abilities, i.e., they can kill a wide range of

abnormal (malignant or virally-infected) cells without having had any previous exposure to the

abnormal cell and/or its antigens. They kill by making cell-to-cell contact with a target cell, then

creating pores in the membrane of the target cell through which toxic granules are delivered,

causing the lysis and/or death of the target cell. Like other cells of the innate immune system,

NK cells are not able to recognize specific antigens on the surface of cells with which they

interact. It was, therefore, a puzzle to immunologists for many years as to how NK cells could

discriminate between abnormal cells that should be killed and normal cells that should be left

alone. It is now known that NK cells make the decision to kill or not to kill by using two types of

cell surface receptors (2). One type of receptor recognizes self MHC molecules on the potential

target cell, and delivers an inhibitory or “don’t kill” signal to the NK cell. A second type of

receptor recognizes a group of stress proteins on target cells, and delivers an activating or “kill”

signal to the NK cell. If a cell is normal, expressing proper levels of MHC molecules, the


inhibitory signal prevents the NK cell from killing that cell. In contrast, abnormal cells that have

been stressed by viral infection, development of malignancy, and/or a rise in temperature express

sufficient levels of stress proteins to successfully engage the activating receptor, while at the

same time having reduced expression of MHC molecules to interact with the inhibitory receptor.

In this case, because there is little or no inhibitory signal given to the NK cell, it acts in response

to the activating signal received, killing the abnormal cell.

NK cells are a very important component of the innate immune system, providing a first

line of defense against virally-infected cells in much the same way that phagocytic neutrophils

serve as the first line of defense against extracellular pathogens. NK cells may also play a role in

protecting the body from cancerous cells. Because they are innate rather than antigen-specific,

NK cells do not have immunologic memory, i.e., are not able to remember and kill the same type

of infected cell more efficiently on a second or subsequent encounter. However, because they are

pre-existing circulating cells with a straightforward recognition and response mechanism, NK

cells respond quickly, effectively eliminating at least some viral infections without any need for

an antigen-specific response. If NK cells are unable to eliminate a viral infection, they play a

vital role in trying to limit the extent of the infection for several days until the adaptive immune

system has been activated to respond. Similar to monocytes and macrophages, activated NK cells

also contribute to the enhancement of both innate and adaptive immune responses by other cell

types through the secretion of cytokines.

The remaining lymphocytes consist of two subsets, B and T lymphocytes, as shown at the

top of Figure 1. It is only these two kinds of lymphocytes that have the capacity to recognize

individual antigens and mount adaptive or antigen-specific responses. Therefore, the ability of

the immune system to respond in an antigen-specific fashion rests completely with the B and T

lymphocytes. For the purposes of further discussion, all future mention of lymphocytes refers

just to B and/or T lymphocytes, not to the non-specific NK cells.

B and T lymphocytes, more commonly referred to as B and T cells, appear identical

when examined under a microscope. For decades, identification and/or separation of B and T

cells required techniques that relied on the different functions of the subsets. Since the

introduction of monoclonal antibodies as reagents for identifying proteins expressed on the

surface of cells, B and T cells subsets are easily identified by characteristic cell-surface markers.

(A more detailed discussion of monoclonal antibodies and cell surface markers follows below.)

However, it is essential to understand the different functions that these lymphocyte subsets

perform.

B cells account for approximately 7-25% of the circulating lymphocytes in the blood

(Table II), and so represent a small minority of the overall circulating WBCs. They were

originally called B cells because they were first described in chickens, where “B cells” mature in

an organ found near the rectum in birds called the bursa of Fabricius (1). Mammals do not have a

bursa of Fabricius, but fortunately for the sake of nomenclature, “B cells” in mammals are both

produced by and mature in the bone marrow.

B cells are antigen-specific cells, capable of binding and responding to individual

antigens, because they carry antigen receptor molecules on their cell surfaces. Just as each

antigen has a distinctive, three-dimensional shape, antigen receptor molecules have a three-

dimensional antigen-binding pocket which is complementary to the antigen that it binds. The

exquisitely specific interaction of an antigen with its specific receptor is like a key fitting into a

lock – only the proper specific antigen receptor can bind to a particular antigen.


Each individual B cell carries thousands of identical antigen receptor molecules on its

surface. This means that any one B cell can recognize only a single antigen, which gives the

immune system great specificity. At the same time, each circulating B cell has a different antigen

receptor, each with a different specificity, which also gives the immune system incredible

diversity. The diversity of the antigen-specific receptors of the immune system has been

described as “virtually limitless” with the ability to generate perhaps hundreds of millions of

different antigen-specific cells (2) – certainly enough to recognize every antigen a person is

likely to encounter in his or her lifetime.

The molecule that serves as the antigen receptor on the surface of B cells is a membrane-

bound form of immunoglobulin (also known as antibody). This is the same protein molecule that

is secreted by B cells when they become antibody-producing plasma cells. Mature but resting B

cells, that have not yet encountered the antigen for which they are specific, make

immunoglobulin molecules, but they are not secreted from the cell. Instead, these molecules

remain attached to the surface of the cell, where they serve as the B cell receptor (also known as

the BCR) for antigen. If and when a B cell encounters the antigen to which its surface

immunoglobulin molecules can bind, the binding of antigen will initiate a complex sequence of

events that can convert the resting B cell into an antibody-secreting plasma cell. This process is

described and discussed in the following course of this immunology overview (3).

B cells are designed to recognize and respond to soluble, extracellular antigens that have

not yet been phagocytosed and processed by an APC. If an antigen is circulating in the blood, it

will likely be trapped by the spleen, where contact with a B cell would occur. Likewise, if a

soluble antigen is introduced into the body via the tissues (which is how most antigen enters),

and is not eliminated by the innate immune response, it will most likely be filtered out by the

lymph system, and will stimulate a B cell response in a draining lymph node. The cell surface

immunoglobulin, which is a Y-shaped molecule, is anchored in the B cell surface by the base of

the Y, with the arms of the Y extending out into the blood or lymph. The antigen-binding

pockets are located on the very tips of the arms, where they are most likely to encounter soluble

antigens. When the proper antigen binds to the antigen-binding pockets on the arms of two

adjacent immunoglobulin molecules (a necessary step known as crosslinking), a signal is

transmitted to the nucleus of the B cell, which begins the process of activation. If the activation

process is successful, immunoglobulin with the same antigen specificity as the antigen receptor

is actively secreted by the newly-developed plasma cell out into the blood and the lymph. This

specific immunoglobulin can then bind to and eliminate the soluble antigen that initiated the

response.

T lymphocytes, or T cells, make up the bulk of the circulating lymphocytes, typically

accounting for 59-86% of the lymphocytes in the blood (Table II). They are called T cells

because immature T cell precursors, which are produced in the bone marrow, go to the thymus to

become fully mature “T cells”. The thymus, a flat bilobed organ located above the heart, is fully

developed before birth, and is the anatomic location where T cells “learn” what is self MHC, and

what are self and foreign antigens. (B cells go through an analogous process while maturing in

the bone marrow.) The thymus is very large in newborn infants, covering the heart, and may

have to be partially removed if neonatal cardiac surgery is required. With age, the T cell-

producing tissue of the thymus is gradually replaced by fat, leaving less than 50% of the original

thymus tissue by adulthood (2). This process, known as thymic involution, does not significantly

impair T cell immunity, as mature peripheral T cells appear to be long lived and/or self-

renewing.


As with B cells, T cells are antigen-specific cells that carry receptors on their cell

surfaces that bind antigen with great specificity. Just as with B cells, all of the thousands of

antigen receptors on a single T cell are identical, capable of binding a single antigen, and each T

cell differs in its antigen specificity. However, unlike B cells, the antigen receptor on T cells is

not cell surface immunoglobulin, and can only recognize cell-associated, not soluble, antigens.

The antigen receptor for T cells, which was discovered many years after the cell surface

immunoglobulin receptor on B cells, is rather unimaginatively called the T cell receptor, or TCR.

It is similar in structure to immunoglobulin, with one end anchored in the cell membrane, and the

other, which contains a single antigen-binding pocket, extending out into the surrounding

medium. It differs significantly from cell surface immunoglobulin, however, in the form of

antigen that it can recognize. In contrast to B cells, T cells are designed to recognize and respond

only to processed, cell-associated antigens. The binding pocket of the TCR binds to a

combination of a digested fragment of an antigen plus a self MHC molecule, as it would be

presented on the surface of an APC (as described above). This combination of processed antigen

plus self MHC is also present on an infected cell, and possibly on a cell that has become

defective in some way. The TCR, therefore, cannot recognize antigen that is still in an intact or

native form. Rather, T cells (via the TCR) only successfully recognize antigen, and transmit an

activation signal to the nucleus of the T cell, if both foreign processed antigen and self MHC are

properly presented to it.

As shown in Figure 1, there is a further subdivision of T cells into two major T cell

subsets, known as CD4+ and CD8+ T cells. CD4 and CD8 refer to two cell surface molecules

that are mutually exclusive in their expression on mature T cells; cells are described as being

“positive” for a cell surface marker when that marker is expressed. T cells express either the

CD4 molecule (CD4+), or the CD8 molecule (CD8+), but not both (under normal

circumstances). These two subsets, while both using TCR as their antigen receptor, also use

either the CD4 or CD8 molecule when physically interacting with processed antigen plus MHC.

This gives rise to an important difference in MHC recognition: CD4+ T cells recognize antigen

plus one type of MHC molecule, Class II MHC, while CD8+ T cells recognize antigen plus

MHC Class I molecules. These differences in CD4 and CD8 expression, and the resulting

difference in antigen plus MHC recognition, are manifested in two distinctly different functions

in the two T cell subsets.

The CD4+ T cells are known as T helper cells (TH), which, as suggested by their name,

provide help to the other antigen-specific lymphocytes (B cells and CD8+ T cells). They

recognize processed antigen presented by APCs in association with Class II MHC. Under normal

circumstances, only certain immune system cells (specifically monocytes and macrophages,

DCs, and B cells) are allowed to express MHC Class II molecules. This ensures that TH

recognize antigen and become fully activated only when they are in contact with the proper cells

of the immune system that are intended to serve as APCs. Hence, monocytes, macrophages,

DCs, and B cells are sometimes referred to as “professional” APCs.

TH provide help primarily through the secretion of cytokines, which deliver critical

signals necessary for the activation and development of B cells into plasma cells, and CD8+ T

cells into cytotoxic T lymphocytes (CTL). These cellular interactions are presented in the

following course of this immunology overview (3). As shown in Table II, there are usually more

CD4+ T cells than CD8+ T cells. This may be expressed as a CD4:CD8 ratio (sometimes called

the “helper:suppressor ratio”), which in a healthy individual is typically about 2:1.


CD8+ T cells, also known as T killer cells or cytotoxic T lymphocytes (CTL), are capable

of killing other cells that are displaying foreign antigen on their surfaces. The manner in which

the target cell is killed is identical to that used by NK cells, i.e., by making cell-to-cell contact,

creating pores in the membrane of the target cell, and causing its lysis and death. However,

unlike the innate NK cells, T cytotoxic cells use their TCR to recognize and kill in an antigen-

specific fashion.

CD8+ T cells recognize and kill cells that are displaying foreign antigen plus self MHC

Class I. When cells are infected with cell-associated pathogens (such as viruses, intracellular

bacteria, or parasites), fragments of foreign proteins belonging to those pathogens become

associated with MHC Class I molecules within the cell. These antigen fragment/MHC I

complexes are then transported to and appear on the infected cell’s surface. In contrast to MHC

Class II molecules on APCs, MHC Class I molecules are not restricted in their expression, and

appear on virtually every nucleated cell in the body. This enables CD8+ CTL to eliminate any

cell in the body that is either infected, or sufficiently defective so as to appear to be foreign.

Since the 1970s, there has been some evidence that there were T cells capable of

suppressing B cell antibody responses and/or T cell cytotoxic responses. However, a distinct

subpopulation capable of suppression could not be identified, and the idea of “suppressor T

cells” fell out of favor. Recently, new cellular and molecular biology techniques have

demonstrated a small naturally-occurring subpopulation of CD4+ T cells that developed under

special conditions in the thymus that can suppress other activated T cells (1). These cells, known

as natural T regulatory (Treg) cells, can inhibit (or down-regulate) other T cells. This regulatory

function appears to be exerted by inhibition of APCs via an antigen-specific interaction

involving the TCR, by direct cytotoxic killing of other T cells, and/or by secretion of inhibitory

cytokines. The primary role of Treg cells is presumed to be the prevention of self-reactive

immune responses, i.e., autoimmunity. Therefore, there is great interest that it might be possible

to increase the activity of Treg cells in an effort to control autoimmune diseases as well as allergic

responses, where the immune system is overactive, and/or to suppress transplant rejection.

Conversely, reducing Treg activity (i.e., alleviating suppression) might improve desirable immune

responses to immunizations and/or tumors. To make matters even more interesting, there are also

“induced” Treg cells, which are T cells that have exited the thymus but take on Treg functions as a

result of exposure to certain cytokines in lymph nodes. They, too, appear to play a role in

preventing autoimmunity. Further characterization and understanding of natural and induced Treg

cells, as well as other newly-defined subsets of T cells such as Th17 cells (which are important

contributors to inflammatory responses), are an exciting area of immunology and clinical

research, but are beyond the scope of this course. III. Essential cell surface molecules

All cells have many different molecules on their surfaces. Some of these molecules are

shared among several cell types, both within and outside of the immune system, while other

molecules are unique to a particular WBC type and/or appear only during certain stages of

maturation or activation. The biological function is known for many well-characterized cell

surface molecules, but remains a mystery for others, that serve purely as markers of certain cell

types (1,2,4). It is important to review a number of cell surface molecules that are essential to the

immune system, either as markers used to distinguish among WBC types or that play critical

roles in the generation of immune responses.


Monoclonal antibodies and the CD nomenclature

The routine use of cell surface molecules as identifiers of different cell types, stages,

and/or function has come about as a result of the development of monoclonal antibodies. A

monoclonal antibody is a preparation of pure antibody molecules, in which every antibody

molecule is identical, so that every antibody in the preparation recognizes and binds to the same

known antigen (1,2). They are powerful antigen-specific reagents that indicate by their binding

(or lack of binding) that the antigen in question is present (or absent) on the surface of cells being

analyzed. Monoclonal antibodies are made by fusing a normal antibody-secreting plasma cell

(that has a limited life span) with a malignant myeloma cell – a cancerous B cell that can grow

indefinitely. The resulting cell, known as a B cell hybridoma, produces antibodies with the same

single antigen specificity as the original normal plasma cell, but can now do so indefinitely. The

B cell hybridoma becomes a perpetual source of an absolutely pure preparation of antibodies

with a single specificity. Once the monoclonal antibodies produced by a hybridoma have been

characterized to identify the antigen recognized, they can be used to determine the presence or

absence of that antigen on cells.

As monoclonal antibodies came into widespread use, considerable confusion arose over

precisely what each antibody preparation was recognizing. Different monoclonal antibodies,

produced by different laboratories and/or companies and each with a different name, appeared to

recognize the same cell surface antigen. As is so often the case when there is confusion over

nomenclature, an international committee was formed to address the rapidly proliferating number

and names of monoclonal antibodies. The committee, in laboratories throughout the world, tested

all the known, widely used monoclonal antibodies available at the time against one another, and

established a standardized nomenclature for the antigens that they recognized (4). This

culminated in 1982, with the first International Workshop on Human Leukocyte Differentiation

Antigens (HLDA), where the term “cluster of differentiation”, or “CD” was introduced.

A cluster of differentiation (CD) refers to a group of monoclonal antibody preparations

that all react with a specific antigen found on a particular cell type or types (1). Therefore, the

focus of the nomenclature is on the antigen that is recognized by one or more monoclonal

antibodies, rather than the name of the individual monoclonal antibody preparation(s). The CD

number of the antigen is used to identify any monoclonal antibody preparation that recognizes

that antigen. For example, as described above, CTLs express the CD8 antigen, and all the

monoclonal antibodies that are known to recognize that CD8 antigen are simply referred to as

“CD8 antibodies”. Any T cell to which a CD8 antibody binds would be considered “CD8-

positive” (CD8+).

As the term HLDA implies, the CD nomenclature was initially applied only to leukocyte

antigens, and only to those antigens that were detectable on the cell surface. Over the years,

however, CD designations have been expanded to include both cell surface and intracellular

molecules on a wide range of cell and tissue types. As of the 10th

HLDA Workshop, held in

December 2014, CD designations were assigned up to CD371 (4). In addition, the name of

molecules described by the CD nomenclature has been changed from Human Leukocyte

Differentiation Antigens to Human Cell Differentiation Molecules (HCDM). The most current

list of CD designations is available online, and provides extensive links to genetic and research

data on CD and related molecules (3). For more general descriptive information about CD

molecules up to CD350, another helpful resource is the Appendix I of the Kuby Immunology

textbook (1).


Cell surface molecules that define different types of WBCs

Most analyses of WBCs from the blood are performed utilizing monoclonal antibodies

tagged with fluorescent labels and a sophisticated laser-based cell counting instrument called a

flow cytometer (a topic that is covered in more detail in the following course in this series). Each

individual cell that passes through the flow cytometer is counted, and evaluated for the presence

or absence of labeled monoclonal antibodies (each antibody labeled with a different color),

which indicates the presence or absence of the CD antigen recognized by the antibodies. The

flow cytometry analysis provides the percentage of cells that are positive for each CD antigen

examined, which can then be used to calculate absolute cell numbers (number of cells/microliter

of blood) of different cell types. Not all clinical laboratories are equipped with a flow cytometer,

so not every Clinical Laboratory Scientist may have a role in performing or reporting the results

of subset determinations. However, it may be helpful to be familiar with the CD designations

that define different types of WBCs.

The most common flow cytometric analysis in a clinical setting is a determination of the

three major lymphocyte subsets—T helper cells (TH), T cytotoxic cells (CTLs), and B cells. NK

cells may also be included in a lymphocyte subset panel, depending on the laboratory and the

clinical and/or research population it serves. The CD antigens utilized for identification of these

subsets, and an example of reference ranges for values obtained from peripheral blood, are

shown in Table II.

B cells express several different CD antigens that can be used to distinguish among B

cells and other types of WBCs (4). However, the expression of some of these antigens changes

with the developmental stage and/or anatomic location of B cells. As shown in Table II, B cells

in the peripheral blood can be identified by the cell surface molecule CD19, which is expressed

on all B cells (except fully mature plasma cells), and is not expressed on any other type of

peripheral blood WBCs (4). Other laboratories may use CD20, 21, and/or 22 for B cell

determinations, depending on the patient population and clinical situations being evaluated.

All T cells, and only T cells, express the CD3 antigen. Since there are two major subsets

of T cells that can be distinguished by the expression of either CD4 or CD8 (as described above),

T cells are usually evaluated with a combination of CD3 plus either CD4 or CD8. TH and CTL

subset analyses are performed by determining the percentage of all lymphocytes that express

CD3 and CD4 (TH) or CD3 and CD8 (CTL). This double expression is important to clearly

identify T cell subsets, as CD4 and CD8 antigens are also expressed on other cell types besides T

cells (1,2,4).

NK cells can be identified by the expression of CD16 and/or CD56, which are not found

on other types of lymphocytes. However, CD16 is also expressed on neutrophils, so NK analyses

using CD16/CD56 must be restricted to lymphocytic (non-granular) cells. Other types of

peripheral blood WBCs can also be characterized by utilizing CD antigens. However, with the

widespread utilization of automated hematology analyzers capable of five-part differentials

(lymphocytes, monocytes, and the three types of granulocytes) based on physical properties

alone, such an analysis utilizing much more expensive monoclonal antibodies and flow

cytometry is not routinely done. For the record, monocytes are non-granular, mononuclear

WBCs that can be identified by the strong expression of CD14, and neutrophils can be identified

by CD16 expression among granular WBCs (4).


Other cell surface molecules critical to immune responses

There are many additional molecules present on the surface of WBCs that are important

to immune system functions, besides those used to identify WBC subsets. As described above,

Class I and Class II MHC molecules on the surface of cells participate in T cell/APC

interactions, and are required for the recognition of self vs. non-self (including transplant

rejection) by the immune system. Likewise, cell surface immunoglobulin (BCR) on B cells, and

TCR on T cells, act as antigen-specific receptors. While the importance of secreted cytokines in

innate and adaptive immune responses was described above, it has not yet been mentioned that

cytokines can only act on cells that express cell surface receptors designed to receive the signal

from that particular cytokine (1,2). Hence, the presence or absence of cytokine receptors on a

cell’s surface will dictate its ability to participate in immune responses, and a large number of

CD designations are dedicated to these receptors. A variety of other cell surface molecules serve

to facilitate cell-to-cell contacts (adhesion molecules), enhance T and/or B cell activation (co-

stimulatory molecules), and regulate cellular proliferation and/or differentiation, but further

discussion of these is beyond the scope of this course.

IV. Beyond the WBCs This course provides an initial insight into the workings of the immune system by

reviewing the first line of defense, the innate immune system, and the cells that contribute to

both innate and adaptive immune responses. It lays the groundwork needed to describe how the

different types of WBCs must interact with each other to recognize and respond to foreign

antigen present in the body, in order to generate the more sophisticated adaptive immune

response. The information presented here also provides the basis for a discussion of clinical

laboratory measurements of immune system function. Both of these topics are presented in a

second course that continues this “Overview of the Immune System” (3). For those interested in

further immunologic adventures, two additional courses are under revision which focus on the

immunology and virology of HIV infection and AIDS.

References 1. Owen, J., Punt, J., Stranford, S. Kuby Immunology, 7

th Ed. W.H. Freeman and Company,

New York. 2013.

2. Parham, P. The Immune System, 4th

Ed. Garland Publishing, New York. 2015.

3. Breen, E.C. An Overview of the Immune System, Part 2: The Generation and Evaluation of

Immune Responses (DL-981, 3.0 CE). California Association for Medical Laboratory

Technology. (Currently under author’s review.)

4. Human Cell Differentiation Molecules, http://www.hcdm.org

http://www.hlda8.org/HLDAtoHCDM.htm


FIGURE 1: The WBCs in peripheral blood


Table I: Typical absolute cell concentrations and frequencies of cell types within total WBCs in peripheral blood (adapted from reference 1)

Cell Type Absolute count, cells/µL

Red Blood Cells (RBCs) 5.0 x 106

White Blood Cells (WBCs) 7.3 x 103

WBC type Absolute count, cells/ µL Frequency, % of total WBCs

Neutrophil 3.7 – 5.1 x 103 50 – 70

Eosinophil 1.0 – 2.2 x 102 1 – 3

Basophil < 1.3 x 102 < 1

Monocyte 1.0 – 4.4 x 102 1 – 6

Lymphocyte 1.5 – 3.0 x 103 20 – 40

Table II: An example of reference ranges and CD markers used in routine flow cytometric lymphocyte subset analyses

Reference Rangesa

Lymphocyte Subset CD(s) Absolute count, cells/µL % of Lymphocytes

Total T cells CD3 841 – 2402 59 - 86

T helper cells CD3, CD4 355 - 1426 29 - 62

T cytotoxic cells CD3, CD8 255 - 1090 16 - 41

B cells CD19 107 - 590 7 - 25

NK cells CD16, CD56 71 - 477 4 - 19

Helper:suppressor ratio CD4:CD8 0.97 – 3.50

a: Data provided as an example from UCLA Health – every laboratory should establish its own reference ranges


REVIEW QUESTIONS Course #DL-978

Choose the one best answer

1. The majority of the components of the innate immune system are:

a) antigen-specific

b) immature cells awaiting activating signals

c) ready to act prior to exposure to antigens

d) part of the major histocompatibility complex

2. Innate immunity utilizes all of the following as barriers except:

a) complement

b) skin

c) lysozyme

d) surface immunoglobulin

3. PAMPs recognized by Toll-like receptors include all of the following except:

a) unmethylated CpG DNA sequences

b) single-stranded viral RNA

c) lipopolysaccharide

d) mammalian DNA

4. Granulocytes, monocytes, and lymphocytes:

a) require flow cytometry analyses to identify them

b) are identical to one another in function

c) play a role only in innate immune responses

d) can be distinguished on the basis of morphology

5. The first WBC that an extracellular pathogen encounters upon entering the body is most likely to be a:

a) NK cell

b) neutrophil

c) lymphocyte

d) monocyte

6. The innate cell that is an important first line of defense against virally-infected cells is a:

a) NK cell

b) neutrophil

c) dendritic cell

d) CTL

7. Monocytes and macrophages:

a) are antigen-presenting cells

b) are cytokine-secreting cells

c) serve as a bridge between innate and adaptive immune responses

d) all of the above


8. Monocytes and neutrophils are both capable of:

a) antigen presentation

b) phagocytosis

c) cell killing

d) immunoglobulin production

9. T lymphocytes are called “T cells” because they:

a) produce cytokines that regulate body temperature

b) mature in the thymus

c) transit through the thyroid

d) are produced from thymic stem cells

10. MHC Class II molecules are normally only allowed to be expressed on the surface of:

a) monocytes/macrophages, dendritic cells, and B cells

b) NK cells and CTLs

c) TH, natural TREG, and induced TREG cells

d) B and T cells

11. TLRs can be found:

a) on the outer surface of a potential pathogen

b) on the outer surface of a human neutrophil

c) inside a human macrophage

d) both a and b

12. The ability to recognize and respond to specific antigens is found in:

a) T cells only

b) B cells, T cells, and NK cells

c) B and T cells

d) basophils

13. Basophils participate in:

a) B cell development

b) transplant rejection

c) CTL responses

d) allergic responses

14. Eosinophils have been observed to be elevated in association with:

a) viral infections

b) bacterial sepsis

c) parasitic infections

d) autoimmune disease

15. NK cells will kill a potential target cell when:

a) antigen receptors recognize foreign antigen plus self MHC on the target cell

b) stress proteins on the target cell deliver a “kill” signal c) reduced levels of MHC molecules on the target cell fail to deliver an inhibitory signal d) both b and c


16. NK cells and CTLs:

a) both kill by making cell-to-cell contact, then delivering toxic granules

b) both utilize a TCR to recognize antigen c) both require cell-to-cell contact with an antigen-presenting dendritic cell d) differ in their killing mechanism due to cytokine secretion by CTLs

17. B cells bind and respond to antigen that:

a) is soluble, especially in the spleen and lymph nodes

b) is presented with MHC molecules on the surface of an APC c) is restricted to the cytoplasm of a cell d) is phagocytosed and digested by a neutrophil

18. T cells recognize and respond to antigen that:

a) is presented with MHC molecules on the surface of an APC

b) is soluble, especially in the spleen and lymph nodes

c) is restricted to the cytoplasm of a cell d) is phagocytosed and digested by a neutrophil

19. The cell surface molecule used as a specific receptor for antigen on T cells is:

a) TLR

b) TCR

c) CD4

d) PPR

20. Under normal circumstances, CD4+ T cells recognize and respond to:

a) foreign antigen plus self MHC Class I molecules

b) foreign antigen plus self MHC Class II molecules

c) self antigens plus any foreign MHC molecules

d) self antigens plus self MHC Class II molecules

21. The function of CD4+ T cells is to:

a) initiate innate inflammatory responses

b) contact and kill virally-infected cells

c) monitor the body for potentially cancerous cells

d) provide help to other cells of the immune system

22. CD4+ T cells perform their function by:

a) recognizing abnormal levels of MHC molecules

b) creating pores in the membrane of the target cell to be killed

c) secreting cytokines

d) secreting antibody to bind and clear soluble antigens

23. CD8+ T cells recognize and respond to:

a) self antigens plus self MHC Class I

b) any cell expressing foreign MHC molecules

c) foreign antigen plus self MHC Class I

d) microbial antigens via TLRs


24. Monoclonal antibodies are:

a) characteristic antibodies found in the circulation of persons recovering from acute

mononucleosis

b) antigen-specific antibody preparations purified from the blood of individuals with myeloma c) genetically-engineered clusters of naturally-occurring antibodies d) a pure preparation of identical antibody molecules secreted by a hybridoma

25. “HCDM” describes:

a) Hybridoma Culture Defined Molecules, a group of cell surface molecules that are recognized

by a single monoclonal antibody preparation

b) the Human Cell Differentiation Molecules categorized by the “CD” nomenclature

c) the Human Classification Designations for Mammalian cells

d) the Hybridoma Committee for Defining Monoclonals, the international committee

responsible for “CD” designations

26. A CD4+ TREG cell in the peripheral blood:

a) is an important contributor to inflammatory responses

b) will co-express CD4 and CD8 on its surface c) is presumed to play a role in preventing autoimmunity d) can suppress NK cell killing

27. B cells can be distinguished from other cells by their expression of:

a) CD371

b) CD56 c) CD14 d) CD19

28. A CD3+, CD8+ cell is:

a) an innate cytotoxic cell

b) capable of antigen-specific cell lysis

c) capable of phagocytosis

d) an immature cell found in the thymus

29. Among the lymphocytes in the peripheral blood, the most plentiful cell type is usually:

a) TH cells

b) CTLs

c) NK cells

d) B cells

30. A cytokine signal sent by one WBC:

a) will stimulate only those cells that are expressing receptors for that particular cytokine

b) can be detected by any other WBC in the vicinity

c) is produced in very large amounts in order to be able to stimulate in a non-specific fashion

d) affects only cells in the immediate area that are expressing TLRs

Lowell

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